Beamforming technology is adopted, for example, in 3GPP standard for TD-SCDMA and LTE. Beamforming is a combination of radio signals from a set of non-directional antennas (adaptive antenna array) to simulate a highly directional antenna. The simulated antenna can be pointed electronically, although the antenna does not physically move. In communications, beamforming is either used to point an antenna array at the signal source to reduce interference, or create the signals towards an intended user, thus improving communication quality. In practice in direction finding applications, beamforming can be used to steer an antenna to determine the direction of the signal source.
Linear antenna array is often used for beamforming, where the distance between the adjacent antenna elements in the antenna array is set to a length proportional to the wavelength of the carrier. For example, half of the wavelength of the carrier is fixed as the distance between two antenna elements in an antenna array. Such a close distance between antenna elements will lead to highly correlated spatial channel seen at the antennas. Grid of Beam (GoB) based beamforming is, for example, often employed as the preferred beamforming technique in this case, but in GoB algorithm, the weighting factor is pre-defined according to the position of the antenna elements due to the high correlation of the antennas.
To reduce the antenna size and facilitate spatial multiplexing, dual polarized antenna array is promoted in TD-SCDMA and TD-LTE deployment. As well known, the correlation between antenna elements of different polarization is very low, the pre-defined weighting vector for single polarized linear antenna array is not suitable for the dual-polarized antenna array. Thus, GoB as a beamforming method is not suitable for the dual-polarized antenna array, since high correlation between two polarization directions is typically not satisfied.
Eigen-based Beamforming (EBB) as another beamforming method is employed for the dual polarized antenna array, and EBB could be done based on infrequent or not overlapping Sounding Reference Signal (SRS) and Physical Uplink Shared Channel (PUSCH). But, the phase difference of the two polarization direction changes faster than the phase difference of the antenna elements within one polarization direction, so for EBB algorithm the phase difference of the two polarization directions should be measured and compensated more often based on SRS or PUSCH.
Furthermore, EBB implies optimal power allocation across antenna elements, which is not practical in the current enhanced NodeB (eNB) building practice where each antenna element has its own power amplifier and power sharing across antennas is not possible, thus, power efficiency could not be well utilized in EBB.
In an existing standard, as there is only one power amplifier in the UE side, UE transmits only on one antenna even if it has multiple receiver antennas. According to the channel reciprocity, only the beamforming weight for one layer corresponding to the ‘active’ antenna can be obtained. Antenna hopping could be enabled in terms of UE capability but which may increase implementation complexity or keep EBB algorithms not-scalable. So, in practice, a full 8×2 channel knowledge at the base station, required in the above EBB scheme, may not be implemented in all cases (here assume 8 antennas at eNB and 2 antennas at UE).
As dual layer beamforming is adopted to improve the throughput for the target. UE by transmitting two layers of beamformed signal. For dual layer beamforming, for example, PMI feedback from UE is used to aid beamforming vector generation in the eNB. But channel accuracy is degraded due to quantification loss. In addition, for fine precoding granularity, PMI may not be satisfied due to limited number of codebooks.